During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the web prior to its entry into the dryer section. As the dryer drums are typically heated from within by steam, costs associated with steam production may be substantial, especially when a large amount of water must be removed from the web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long press nips of the shoe type has been found to be more advantageous than the use of nips formed by pairs of adjacent press rolls. This is because the longer the time a web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the web for removal through evaporation in the dryer section.
The present invention relates, in part, to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity to one another, a nip, which can be five to ten times longer in the machine direction than one formed between two press rolls, is formed. Since the long nip may be five to ten times longer than that in a conventional two-roll press, the so-called dwell time, during which the fibrous web is under pressure in the long nip, may be correspondingly longer than it would be in a two-roll press. The result is a dramatic increase in the dewatering of the fibrous web in the long nip relative to that obtained using conventional nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in U.S. Pat. No. 5,238,537 to Dutt (Albany International Corp.), the teachings of which are incorporated herein by reference. The belt is designed to protect the press fabric, which supports, carries and dewaters the fibrous web, from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be provided with a smooth, impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
Belts of the variety shown in U.S. Pat. No. 5,238,537 are made by impregnating a woven base fabric, which takes the form of an endless loop, with a synthetic polymeric resin. Preferably, the resin forms a coating of some predetermined thickness on at least the inner surface of the belt, so that the yarns from which the base fabric is woven may be protected from direct contact with the arcuate pressure shoe component of the long nip press. It is specifically this coating which must have a smooth, impervious surface to slide readily over the lubricated shoe and to prevent any of the lubricating oil from penetrating the structure of the belt to contaminate the press fabric, or fabrics, and fibrous web.
The base fabric of the belt shown in U.S. Pat. No. 5,238,537 may be woven from monofilament yarns in a single or multi-layer weave, and is woven so as to be sufficiently open to allow the impregnating material to totally impregnate the weave. This eliminates the possibility of any voids forming in the final belt. Such voids may allow the lubrication used between the belt and shoe to pass through the belt and contaminate the press fabric or fabrics and fibrous web. The base fabric may be flat-woven, and subsequently seamed into endless form, or woven endless in tubular form.
When the impregnating material is cured to a solid condition, it is primarily bound to the base fabric by a mechanical interlock, wherein the cured impregnating material surrounds the yarns of the base fabric. In addition, there may be some chemical bonding or adhesion between the cured impregnating material and the material of the yarns of the base fabric.
Long nip press belts, such as that shown in U.S. Pat. No. 5,238,537, depending on the size requirements of the long nip presses on which they are installed, have lengths from roughly 10 to 35 feet (approximately 3 to 11 meters), measured longitudinally around their endless-loop forms, and widths from roughly 6 to 35 feet (approximately 2 to 11 meters), measured transversely across those forms. The manufacture of such belts is complicated by the requirement that the base fabric be endless prior to its impregnation with a synthetic polymeric resin.
It is often desirable to provide the belt with a resin coating of some predetermined thickness on its outer surface as well as on its inner surface. By coating both sides of the belt, its woven base fabric will be closer to, if not coincident with, the neutral axis of bending of the belt. In such a circumstance, internal stresses which arise when the belt is flexed on passing around a roll or the like on the paper machine will be less likely to cause the coating to delaminate from either side of the belt.
Moreover, when the outer surface of the belt has a resin coating of some predetermined thickness, it permits grooves, blind-drilled holes or other cavities to be formed on that surface without exposing any part of the woven base fabric. These features provide for the temporary storage of water pressed from the web in the press nip, and are usually produced by grooving or drilling in a separate manufacturing step following the curing of the resin coating.
The present invention relates particularly to a grooved press belt which could be used in a long nip press or also in a conventional press. In addition, the present invention relates to a grooved roll cover.
In the case of conventionally grooved belts, a machining process is typically used to create the surface grooves. The grooves define channels and are separated from one another by what may be referred to as land areas. The width and depth of the grooves and the number of grooves per inch dictate the void volume of the grooved belt surface. In addition, the need for the belt to flex dictates certain material characteristics. For instance, the material must be sufficiently flexible to conform to the belt path and still be rigid enough so as not to collapse under press loads. In this regard, it is noted that elastomers such as urethane have been successfully used heretofore.
Roll covers are manufactured in a variety of ways, using varied materials such as rubber, rubber-like materials, polymers or metal alloys. Current methods of manufacturing roll covers include “laying up” sections of uncured mats of polymeric compounds onto a roll body, sometimes with textile reinforcement. This forms a roll cover of several sections, which is finished by heat curing to form a continuous cover. This cover is ground and may undergo other surface finishing steps. Finally, grooves are cut in a circumferential pattern in the surface to assist sheet dewatering in the press nip. Another method used to form roll covers is to spiral wind a semi-solid resin onto a body, followed by curing and surface finishing. Again, grooves are cut in the surface to assist dewatering in the nip. Yet another method is to cast or mold a resin-fiber system onto a mandrel or roll body, forming a composite system. Other covers may be formed of mixtures of resins (i.e., “alloys” of resins), metals and resins, ceramics, and the like.
Also in the case of conventional grooved rolls, the surface grooves are typically created by machining the roll cover surface. Again, the width and depth of the grooves and the number of grooves per inch dictate the void volume of the grooved surface. It is also noted that there are many different materials used to create grooved roll surfaces.
During the machining of grooves especially in belts, the interior walls (the sides of the lands separating the grooves) of synthetic surface material are left with microscopic surface cuts created by the action of the machine tool. These microscopic cuts can constitute crack initiation sites which may lead to larger cracks and eventual failure or delaminating of the land area on either side of the groove. In this connection, U.S. Pat. No. 5,171,389 is directed toward a method of making a grooved void-volume belt for use on a long nip press. After applying an adhesive, a strip of material having a groove already formed therein is wound about a partially completed belt and suitably attached. This, among other things, avoids having to cut grooves in the belt surface.
The present invention provides an approach towards forming grooves or other patterns on both belt and roll surfaces without machining using ribbon placement to create the grooves.